The present invention relates to high tenacity acrylonitrile (hereinafter abbreviated as AN) fibers, particularly to novel AN fibers which comprise an AN polymer having a high degree of polymerization and having an extremely high mechanical strength, a remarkable smoothness at the surface thereof and a dense and homogeneous fiber structure, as compared to conventional and commercially available AN fibers. The present invention also relates to hydraulic substances reinforced with these fibers and a process for production thereof.
AN fibers have been hitherto produced and sold for use for clothing items in large quantities. However, it is an actual circumstance that little AN fibers have been employed for use as industrial materials because of unsatisfactory mechanical strength.
Many attempts to enhance or improve the mechanical properties of AN fibers have been proposed heretofore.
For example, Japanese Patent Publication Nos. 19414/70 and 29891/71 propose a process for producing a high tenacity acrylic fibers for use as industrial materials by introducing a solution of AN polymer into a coagulating bath through an inert gaseous medium to coagulate, namely a dry-jet wet spinning method, which comprises forming coagulated threads by a dry and wet spinning process, subjecting the fibers to washing, hot drawing, an after-treatment and drying, then secondary drawing and further subjecting to a heat treatment under a shrinkage allowance, particularly under the conditions such as above-described two-stage drawing and high speed winding.
However, as shown in Examples of the above Patent Publications, the strength of fibers produced by the above process is only of such a level as to be 3.2 to 4.9 g/d, and the fibers can hardly be utilized as industrial materials.
Further, a process which comprises subjecting fibers obtained by a wet or dry spinning method to wet drawing, drying under a tension, and subsequently subjecting to contact drawing to make an effective total drawing ratio of 9 to 25 times is proposed in Japanese Patent Application KOKAI (=laying-open) Publication No. 51810/82. It is described therein that high modulus AN fibers can be obtained by this process.
However, according to the process, the contact drawing is made subsequent to spinning by a wet spinning or dry spinning, and the level of strength of fibers obtained is so low as to be 9.2 g/d for the highest strength value of fibers obtained in the Examples of the Publication. Also, as clearly seen from the fact that the smoothness of the fiber surface is so poor that the greatest knot strength obtained in the Examples is 1.5 g/d, the fibers have only a low knot strength and are not much useful for practical utility as industrial use fibers.
Furthermore, a process which comprises subjecting a polymer having a relative viscosity of 2.5 to 6.0 to a dry or wet spinning method, washing or washing followed by wet drawing, drying on a heating roll in a tension state, drawing under dry heating and then heat-treating is proposed in Japanese Patent Application KOKAI Publication No. 161117/82; it is described therein that high tenacity AN fibers can be obtained by this process.
However, the strength of AN fibers obtained by the prior art is, for example, less than about 10 g/d at best in tensile strength. In addition, as the molecular weight or degree of polymerization of AN polymer employed increases, the AN fibers obtained often causes defects such as a mutual adhesion between the fibers etc. As a result of the defects, the AN fibers do not enhance or improve not only a tensile strength but also other mechanical properties such as an initial modulus, a knot strength, a heat resistance, etc.
Then, U.S. Pat. No. 4,535,027 discloses a process for producing a high strength AN fiber having a tensile strength of 20 g/d or above by way of a dry-jet wet spinning and a multi-stage drawing.
According to this process, however, use is made of an AN having a weight average molecular weight of 400,000 or above, so that the process involves a difficulty such that the viscosity of the spinning solution tends to be so high that it becomes extremely difficult to operate the spinning.
In practice, now that fibers are produced by drawing to a high draw ratio exceeding 25 times, a sufficient degree of the molecular orientation can be obtained in the fiber axial direction but the fibers tend to be insufficiently strong in the direction perpendicular to the fiber axial direction. That is to say, fibers are prone to undergo fibrilation when subjected to friction, are relatively poor in the knot strength, the frictional abrasion resistance and so forth so that they are poor in the practical usefulness as reinforcing fibers in or for industrial material and composite material.
On the other hand, asbestos has been hitherto used as reinforcements of hydraulic substances such as cement or gypsum. Methods for producing asbestos-reinforced plane plates, corrugated plate slates, etc. include wet paper-making methods of cylinder mould type and fourdrinier type. Among them, a cylinder mould type paper-making method called a Hatcheck's method has received a reputation as a preferable method for past several ten years. The asbestos fibers for reinforcing cement have a very good affinity and a strong adhesion force to cement matrix. The presence of long fibers with short fibers in an appropriately mixed state enhances an efficiency of paper-making and makes a reinforcing effect satisfactory and, the asbestos fibers are thus ideal fibers for use in cement reinforcement.
However, from a situation that harmful nature of asbestos fibers in handling becomes a problem on a world-wide basis, it has been actually expected to develop substitute materials for asbestos. As substitute materials for asbestos fibers, inorganic and organic fibers such as glass, polyethylene, polypropylene, nylon, polyacryl, polyvinyl alcohol, carbon, aramide, alumina fibers, etc. have been attempted to be utilized.
In order to obtain a cement plate having an excellent reinforcing effect and durability in a papermaking method, the following requirements should be met, namely:
(1) Fibers having a small diameter be separated from each other without being entangled and uniformly dispersed in a cement suspension (slurry).
(2) Affinity of fibers to cement which greatly affects a paper-making property and a reinforcing effect be good and adhesion between fibers and a cement be strong.
(3) Durability, particularly alkali resistance, be excellent; etc.
However, the organic and inorganic fibers which have been attempted to be utilized heretofore did not satisfy all of the requirements described above.
For example, fibers such as polyethylene, polypropylene, nylon, etc. only have poor strength and poor tensile modulus. Further, adhesive force to cement matrix is weak. Thus, no satisfactory reinforcing effect can be obtained. Glass fibers have a poor alkali resistance and an unsatisfactory adhesion force. Further, aramide fibers and carbon fibers have poor dispersibility, weak adhesion force and are costly. Therefore, these fibers have not been adopted yet. Further, polyvinyl alcohol fibers and acrylic fibers promise a bright future as fibers substitutable for asbestos because alkali resistance is good and adhesion to cement matrix is strong. However, polyvinyl alcohol fibers encounter a problem in costs. Acrylic fibers have a poor tensile strength and a poor initial modulus; accordingly, hydraulic substances having high efficiencies such as slates having a high bending strength, etc. cannot be obtained from these fibers.
On the other hand, various methods for improving efficiencies, e.g., bending strength, etc., of hydraulic substances reinforced with the aforesaid acrylic fibers have been proposed in recent years. For example, in Japanese Patent Application KOKAI Publication No. 170869/82 (U.S. Pat. No. 4,446,206), there is disclosed a hydraulic substance reinforced with acrylic fibers which contain 98 to 100% of acrylonitrile and have a tensile strength of at least 50 CN/tex (5.65 g/d) and a tensile elongation of at most 15%.
However, acrylic fibers concretely described in the above-described publication are all obtained by subjecting acrylonitrile type polymers to wet spinning. The thus obtained fibers merely possess a tensile strength of at most 85 CN/tex (9.63 g/d) and a tensile modulus of at most 1510 CN/tex (171.1 g/d). A bending strength of cement reinforced with these acrylic fibers is considerably inferior to that of conventional asbestos-reinforced cement. In addition, a bending strength of a cement reinforced with acrylic fibers showing the highest tensile strength of 9.63 g/d is not necessarily large but a relatively considerably low bending strength is merely obtained.
The utility is only limited, of such fibers as having a strength of the above-mentioned level. Also, as before stated, according to the above methods fibers are produced by a wet spinning, so that the fibers have defects on the surface condition or fail to have a desirable surface smoothness, and because of this, the fibers are relatively low in the knot strength and prone to fibrilation when subjected to friction of abrasion, and are relatively low also in the toughness. For example, when the fibers are used as reinforcing fibers in cement, it is difficult to obtain a cement product having desirable properties such as for example a desirably high impact strength.
Further, hydraulic substances reinforced with conventional asbestos are brittle due to remarkably small work load of rupture in bending and low impact strength, which often causes damages in building and construction sites.
Furthermore, it is generally observed that an impact strength of hydraulic substances reinforced with synthetic fibers such as acrylic fibers, etc. described above would be improved. However, the improvement is unsatisfactory yet. It has thus been desired to develop hydraulic substances having not only an improved bending strength but also a high impact strength.
On the other hand, various methods for enhancing an cohesion between fibers and cement particles in a slurry state of cement and improving a paper-making property have also been proposed. For example, in Japanese Patent Application KOKAI Publication No. 62833/80, there is disclosed a process for producing a cement plate which comprises adding 10 to 800 ppm of a flocculant selected from strong anionic, medium cationic and weakly cationic flocculant to a cement slurry having formulated no asbestos fibers and then subjecting a cement plate to paper-making.
However, cohesion between fibers and cement particles is hardly achieved and fibers are easily separated from cement particles b a shearing force at a paper-making step to cause heterogenecity of the slurry, while improvement in a paper-making efficiency is expected to a certain extent in this process because flocs of cement particles are formed by the addition of the flocculant and thus, outflow of cement particles from a wire cylinder is prevented at the paper-making step. After all, no improvement in property of the thus obtained cement plate after molding can be expected.
Furthermore, a process for reinforcement of cement materials using polyvinyl alcohol synthetic fibers having coated thereon 0.01 to 3 wt % of anionic or/and nonionic surfactants and a cationic oil is disclosed in Japanese Patent Application KOKAI Publication No. 134553/81. Further in Japanese Patent Application KOKAI Publication No. 13455/81, a process which comprises using a cationic oil and a nonionic or amphoteric surfactant in combination as a treating agent is disclosed.
However, when these processes are used, these processes encounter a drawback that due to unsatisfactory paper-making efficiency, namely, due to unsatisfactory cohesion between fibers and cement particles in a slurry state, paper-making cannot be performed efficiently while an effect of improving adhesion between fibers and cement particles in cement articles, that is, an effect of improving bending strength, can be expected to a certain extent.
As described above, in the case of preparing cement plates through a paper-making process, it is important that cement particles be firmly fixed on the surfaces of fibers in large quantities at a paper-making step, a uniformly dispersed slurry be obtained and, at the same time, a felt material having good uniformity be formed by a cylinder mould machine or a fourdrinier machine while maintaining the firmly fixed state and the uniformly dispersed state. In the prior art processes, however, cohesion between fibers and cement particles in a slurry state or dispersibility of the slurry is insufficient so that paper-making efficiency is not always satisfactory. As a result, it has been prevented to improve properties of cement plate.